I assume that you have started the game and see the timer running?

You can click the Volume icon on the Windows Taskbar near the clock and click the Mixer link at the bottom to open the mixer.
You can adjust the volume level for each application in that window, so make sure that Firefox is not muted or set at a low level.
Firefox uses the plugin-container process to run plugins, so you need to check the volume setting for the plugin-container process.
You will only see the Firefox program or the Plugin Container in the Mixer if you are on a website that plays sound.

See also:

• https://support.mozilla.org/kb/fix-common-audio-and-video-issues

Thanks cor-el,

Firefox was muted in my Mixer. I'm on a laptop, is there some kind of shortcut or key that you can press to mute Firefox in the Mixer? Or is the only way to mute Firefox in the Mixer is to click mute with the mouse?

I did not know that you can adjust the volume level for each application, is this a new feature that was introduced with Windows 8?

No, this feature has been there since Windows Vista (Windows XP only has one global volume setting).
I'm not aware of a way to mute the sound apart from doing this in the volume mixer.